We propose to define more precisely the relationship among the various flow indicators with reference to their distribution in the different regions of the heart. The flow indicators are; entrapped carbonized microsphere of size 9 and 35 u in diameters, flow-limited iodoantipyrine, and partially flow-limited potassium chloride. These will be injected simultaneously and their distribution profile compared. The animals preparations are either isolated blood perfused Langendorff dog hearts or hearts in open-chested animals. These studies will be done under various physiological and pharmacological interventions to produce abnormal subendocardial perfusions. The aim is to define the best indicator for regional myocardial blood flow, and once that is known, this indicator will be used to define the physiologic or pharmacologic factors which determine the regional myocardial flow. BIBLIOGRAPHIC REFERENCES: T. Nivatpumin, T. Yipintsoi, S. Penpargkul and J. Scheuer. Increased cardiac contractility in acute uremia: interrelationship with hypertension. Am. J. Physiol. 229: 501-505, 1975. B. Guller, T. Yipintsoi, A.L. Olvis and J. B. Bassingthwaighte. Myocardial extraction of sodium at various coronary flows in the dog. Circ. Res. 37: 359-378, 1975.